Brian "Psychochild" Green recently wrote up a post on the problems of innovation in MMOs, as well as several reasons we don't tend to get a great deal of it. In short, there are two main reasons. Innovation is risky, which makes a company more and more wary of investing money in a game that might not make any of it back. It also goes directly against another virtue that we look for in our games: polish. By its very definition, an innovative game can't be polished, since there's not as much precedent for refining systems.
It's hard to summarize the full essay in a few sentences, however, as it covers a lot of ground and a wide variety of games. If you're interested in why it seems like there's little new under the MMO sun, you could do worse than reading the full thing through.